intro -- a brief introduction to the CL
The CL (or Command Language) is the command interpreter of the IRAF environment. Among it's responsibilities are: task initiation and termination; parameter retrieval and updating; and error handling. In addition the CL has certain `builtin' utility functions which enable monitoring and changing of the IRAF environment, control flow features roughly modeled after C and Ratfor as well as fairly sophisticated capabilities for performing mathmatical calculations and string manipulation. The CL environment may easily be extended by the user.
2. Task Initiation and Termination
IRAF organizes tasks into groups called packages . When a package (which is itself a special kind of task) is invoked, it defines all the tasks which belong to that package, and the user may then execute any of the tasks in the package. Some of these new tasks may themselves be packages. Normally at the start of a CL session, the language package, including all functions built directly into the CL, and the system package, which contains basic system utilities, are automatically invoked. The user may configure their "login.cl" file to automatically invoke other packages.
Within the CL a task is invoked by entering its name, e.g.
cl> reduce args
If two tasks in different packages have the same name, then the package name may be included:
cl> spectra.reduce args
The task name may be followed by a parameter list and tasks may be linked together by pipes (see parameters). The task initiates execution of either a script file, an ASCII file containing further CL commands, or an executable image, an external program linked with IRAF libraries so that it may be called as a sub-process from the CL process. The correspondence between the task name and the name of the script or image file is made using the task and redefine builtin commands.
When a script is run the CL effectively calls itself recursively with the new incarnation of the CL having its standard input not from the terminal, but from the script file. When the script terminates the recursion unwinds, and the CL returns to an interactive mode. A script may itself call another script or executable.
An executable is run as a separate process with communication and synchronization maintained using an inter-process communication link (a pipe in UNIX or a shared memory region in VMS). When the executable requires a parameter a request is sent across the link, and the CL replies in the same fashion. When the process terminates it informs the CL and then hibernates. Normally the executable's process is not terminated, but is maintained in a process cache so that the executable may be used again without the overhead of reinitiating the process. The process finally terminates when the CL finishes, when the space in the cache is needed by a new executable image, or when the user explicitly clears the cache using the flprcache command. The size of the process cache is small, usually only three executables can be maintained in the cache.
3. Parameter Retrieval and Update
Most CL tasks have a parameter list associated with them. When the task starts up, the CL looks to see if the user has a private copy of the parameters from the last time he ran this task. If so these parameters are loaded into memory. Otherwise the CL looks for the default values of the parameters and loads these. While the task is active the parameters are maintained in memory, but when it finishes the CL checks if any `learned' parameters have been modified. If so a new private copy of the parameters is stored into the directory pointed to by the IRAF logical name `uparm'. A number of builtins are used to control the handling of parameters including lparam, eparam, update and unlearn.
4. Error Handling
The CL attempts to trap most kinds of errors that may occur and to keep the user in a viable IRAF environment. When an error occurs in a script, execution of the script is terminated and the CL returns to an interactive level. The user may force an error using the error builtin. When a executable image encounters an error it cannot handle itself, it sends an error message to the CL and then hibernates in the process cache until its next invocation. If executable was called by a script, the script is terminated and the CL returns to an interactive mode. The error message from the executable is relayed to the user.
commands, mathfcns, strings